The present invention relates generally to an angle modulation system, and more particularly to an angle modulation system with a large modulation index, using a phase locked loop.
An example of an angle modulation system in which a phase locked loop (hereinafter referred by the abbreviation PLL) is used, has been known in the prior art. This system comprises a voltage controlled oscillator (hereinafter referred to by the abbreviation VCO), a reference frequency oscillator, a phase comparator supplied with the output of the VCO and the reference frequency of the reference frequency oscillator, and an adder for adding the output of the phase comparator and an input modulation signal. The resulting output is supplied to the VCO. In this known system, the VCO, the phase comparator, and the adder constitute a PLL. The VCO accomplishes angle modulation in accordance with the input modulation signal. The oscillation of the VCO is controlled in accordance with the phase difference or the frequency difference between the output of the VCO and the reference frequency. As a result, an angle-modulated signal is produced as the output of the VCO.
The phase comparison limiting value of a phase comparator is, in general, .+-. .pi./2 radians (rad.) when a phase comparator employs a multiplier, and .+-. .pi. rad. or .+-. 2.pi. rad. when there is a digital phase comparator. In either case, the phase comparison limiting value of the phase comparator is in the order of .+-. 2.pi. rad., as a maximum. On the other hand, the lock range of a PLL is restricted by the phase comparison limiting value of the phase comparator. Consequently, the modulation index of the angle modulation is restricted by the phase comparison limiting value.
In this known angle modulation system, therefore, the limit of the angular deviation is .+-. 2.pi. rad., even when a phase comparison limiting value of .+-. 2.pi. rad. is used. As a consequence, an angle modulation having a deviation exceeding this value cannot be carried out, and a large modulation index cannot be used.
In order to overcome the above described drawbacks, the following system has been proposed. This proposed system comprises a VCO, a frequency divider for dividing to 1/N the frequency of the output signal of the VCO, a reference frequency oscillator, another frequency divider for dividing to 1/N an oscillation reference frequency of the output of the reference frequency oscillator, a phase comparator supplied with the outputs of the above mentioned two frequency dividers. An adder adds the output of the phase comparator and an input modulation signal, and, supplies the resulting output to the VCO.
When the angle-modulated signal produced by the VCO is divided to 1/N, the angle deviation is also divided to 1/N. For this reason, if the 1/N divided angle deviation is made equal to the phase comparison limiting value (.+-. .pi. rad.) of the phase comparator, the angle deviation of the angle-modulated wave can be .+-. 2.pi. .times. N rad. That is, the maximum modulation index can be made N times that in the above first described system of the prior art.
However, when the frequency dividing ratio N, of the above mentioned frequency divider, is, made a large value to attain a desired maximum modulation index, the frequency of the input signal introduced into the phase comparator decreases by that much. If this frequency of the input signal into the phase comparator is a frequency which is within the frequency band of the modulation signal, beats will be produced at a certain frequency within the modulation signal.